This invention generally relates to display media, and the preparation of display media, and to devices using such display media. In particular, this invention relates to display media and displays for which the image remains in view after the field and/or power used to form the image is eliminated (completely reduced to zero), or reduced (decreased to a level below normally required to form the image). The image is formed by switching materials in the pixels between two states (such as, for example, black and white). In embodiments, the display media comprise bichromal beads, and in preferred embodiments, Gyricon beads. In embodiments, the bichromal beads comprise electrolytes dispersed or contained therein. In embodiments, the electrolytes are polymer electrolytes. The display media made with Gyricon beads are useful in generating images which can be stored or erased, and function by rotating a bichromal sphere by an external field to create the image.
Display media, such as Electric Paper or twisted ball panel display devices, are known and are described, for example, in U.S. Pat. Nos. 4,126,854; 4,143,103; 4,261,653; 4,438,160; 5,389,945. The media generally are comprised of a substrate material, for example, an elastomer, such as a cured polysiloxane, sandwiched between two indium tin oxide coated substrates, such as glass or MYLAR(trademark). Generally, the elastomer layer has closely packed cavities, each containing a bichromal sphere suspended in a dielectric liquid. The dielectric liquid may also be present in substantial amounts in the elastomer matrix. In media that are active in an electric field, the bichromal spheres have a net dipole due to different levels of charge on the two sides of the sphere. An image is formed by the application of an electric field to each pixel of the display, which rotates the bichromal spheres to expose one color or the other to the viewing surface of the media. The spheres may also have a net charge, in which case they will translate in the electric field as well as rotate. When the electric field is reduced or eliminated, the spheres ideally do not rotate further; hence, both colors of the image remain intact. This image bistability is one feature of display media made with bichromal Gyricon beads.
The fabrication of certain bichromal spheres is known, for example, as set forth in the above mentioned U.S. Pat. No. 4,143,103, wherein the sphere is comprised of black polyethylene with a light reflective material, for example, titanium oxide, sputtered on hemisphere. Also in U.S. Pat. No. 4,438,160, a rotary ball is prepared by coating white glass balls of about 50 microns in diameter, with an inorganic coloring layer such as indium by evaporation. In a similar process, there is disclosed in an article entitled xe2x80x9cThe Gyriconxe2x80x94A twisting Ball Displayxe2x80x9d, published in the proceedings of the S.l.D., Vol. 18/3 and 4 (1977), a method for fabricating bichromal balls by first heavily loading glass balls with a white pigment such as titanium oxide, followed by coating from one direction in a vacuum evaporation chamber with a dense layer of nonconductive black material which coats only one hemisphere.
Also in U.S. Pat. No. 4,810,431 by Leidner, there is disclosed a process for generating spherical particles by (a) coextruding a fiber of a semi-circular layer of a polyethylene pigmented white and a semi-circular black layer of polyethylene containing magnetite, (b) chopping the resultant fiber into fine particles ranging from 10 microns to about 10 millimeters, (c) mixing the particles with clay or anti-agglomeration materials, and (d) heating the mixture with a liquid at about 120xc2x0 C. to spherodize the particles, followed by cooling to allow for solidification.
Reference is made to U.S. Pat. No. 5,262,098, and in co-pending patent applications Ser. No. 09/360,088, filed Jul. 23, 1999, entitled xe2x80x9cMethod and Apparatus for Fabricating Bichromal Elementsxe2x80x9d, and Ser. No. 09/360,052, filed Jul. 23, 1999, entitled xe2x80x9cMethod and Apparatus for Fabricating Bichromal Elements.xe2x80x9d These applications disclose apparatuses for fabricating hemispherically bichromal balls comprising a separator member having opposing first and second surfaces and an edge region in contact with both surfaces, and delivery means for flowing first and second colored hardenable liquid material over the first and second surfaces, respectively, so that the liquid materials arrive at the edge, usually at substantially the same flow rate, and form a reservoir outboard of the edge region. The reservoir comprises side-by-side regions of different colors, which in a preferred embodiment, do not intermix. Further means are provided for propelling the first and second liquid materials away from the separator member and out of the reservoir into a fluid medium. As this occurs, a plurality of forward ends of side-by-side bichromal streams become unstable and break up into droplets. The droplets form into spherical balls, each of the balls approximately comprising hemispheres of differently colored hardenable liquids. These bichromal balls are from about 5 to about 200 microns in diameter.
The aforementioned display media can suffer from drawbacks caused by incomplete rotation of the bichromal beads. When the beads do not rotate close to 180xc2x0, the switching from one color to the other is not complete. As a result, image quality suffers. In some cases, increasing the strength of the electric field used to rotate the spheres can help in achieving more complete rotation, but in other cases sufficient rotation cannot be attained, even at higher fields. In the latter cases, it is believed that the dipole strength of the sphere relative to the monopole strength is too small, rendering it difficult to get sufficient rotation before the sphere translates across its cavity in the elastomer matrix. Many of the beads may even lack sufficient monopole and dipole strengths to dislodge them from the cavity walls. Furthermore, it is usually preferable to produce media requiring an electric field that is not too high in magnitude, since the cost, robustness, and power consumption of display products made from media that switch at lower electric fields can be advantaged.
Another drawback of the aforementioned display media is the lack of a sharp voltage threshold. Consider a media that requires a voltage xc2x1V to get sufficient sphere rotation and hence switching between colors. There is a sharp voltage threshold above a magnitude of xc2xdV when the spheres do not rotate at voltages of magnitudes less than or equal to xc2xdV. During the course of writing an image on a media with a sharp voltage threshold above a magnitude of xc2xdV, pixels that experience voltages between xe2x88x92xc2xdV and +xc2xdV will not change their color. For certain applications this property is desired. An example application is a display device with passive matrix addressing. In the case of passive matrix addressing, a pixel of the display is addressed by applying half of the required voltage to both the row and column of that pixel, the two half voltages having opposite polarities to yield a total voltage across the pixel equal to the switching voltage of xc2x1V. At the same time, however, other pixels in the same row but in other columns, or in the same column but in other rows, experience a voltage of xc2x1xc2xdV. Thus, it is desired that the spheres in those pixels do not rotate at the voltage xc2x1xc2xdV. A sharp voltage threshold above xc2x1xc2xdV gives the desired behavior, whereas a media that lacks a sharp voltage threshold does not. Since the display media described in the above paragraphs may not have a sharp voltage threshold at or above xc2x1xc2xdV when made using bichromal spheres according to the examples and formulations disclosed in the prior art, passive matrix addressing on such media results in poor image quality due to rotation of spheres at voltages in the range of xe2x88x92xc2xdV to +xc2xdV.
Materials that can improve the rotational behavior of bichromal beads could enable display media to be used in a wider variety of applications than is currently possible. For example, materials that provide a more reproducible and lower voltage for rotation and a sharper voltage threshold above one-half of the voltage used for rotation, could be used to make bichromal passive matrix displays a reality. Therefore, it is desired to provide a display media wherein a threshold voltage exists that is larger than one-half of the voltage used for rotation. It is further desired to provide a display media wherein the threshold voltage is sharper to eliminate most, or ideally all, of the rotation below the threshold voltage and more complete rotation can be obtained at a lower applied voltage.
The present invention provides, in embodiments, for bichromal sphere formulations that include electrolyte materials in order to help achieve lower switching voltages, faster and more complete rotation of beads, and more distinct voltage thresholds for displays and display media containing bichromal beads, and in particular, Gyricon beads.